Formula (1):

Tazobactam, which is represented by Formula (1) given above, exhibits very weak antibacterial activity, and it is therefore not used alone as an antibacterial agent. However, it irreversively binds to various β-lactamases produced by microorganisms and exhibits an ability to inhibit β-lactamase activities. Hence, tazobactam is used in combination with various existing antibacterial agents that are inactivated by β-lactamases, allowing such antibacterial agents to exhibit their inherent antibacterial activity against β-lactamase-producing microorganisms (Katsuji SAKAI, Recent Antibiotics Manual, 10th ed., page 113).
As shown in the reaction scheme below, tazobactam is produced by oxidizing 2α-methyl-2β-[(1,2,3-triazol-1-yl)methyl]penam-3α-carboxylic acid benzhydryl ester (hereinafter sometimes referred to as “TMPB”) and de-esterifying the resulting 2α-methyl-2β-[(1,2,3-triazol -1-yl)methyl]penam-3α-carboxylic acid 1,1-dioxide benzhydryl ester (hereinafter sometimes referred to as “TAZB”) thus obtained. Therefore, TMPB is of use as an intermediate for synthesizing tazobactam and as a precursor of TAZB.

Since the nucleophilic 1,2,3-triazol moiety is contained in the TMPB molecule, oily or amorphous TMPB is unstable and likely to undergo decomposition, degeneration, etc. For this reason, efforts have been made to isolate crystalline TMPB, which is stable (WO02/14325).
The method disclosed in WO02/14325 produces TMPB crystals by concentrating a TMPB-containing solution, diluting the concentrated solution with an acetic acid ester, and mixing the diluted solution with hexane or like solvent.
However, in the method disclosed in WO02/14325, the efficiency of separating TMPB from by-products that are simultaneously generated in the reaction is low. Therefore, to obtain highly pure TMPB crystals, large amounts of TMPB inevitably remain in the mother liquor, resulting in a low yield of TMPB crystals.